The invention relates to a control device and a method for the setting/closed-loop control of a cross-machine and/or machine direction profile of a quality feature in the paper manufacturing process.
Control systems which have been used for the cross-machine and/or machine direction profile of a quality feature in the paper manufacturing process. For example, the grammage was controlled as the quality feature. The grammage of the running paper web was measured at the papermaking machine by a sensor, for example a traversing sensor. The measured variable was read into an open-loop/closed-loop control device to control the cross-machine direction profile. Through a control program, the open-loop/closed-loop control device drove actuators to influence the cross-machine direction grammage profile. An open-loop/closed-loop control device of this type and a method of controlling the grammage and fiber orientation cross-machine direction profile are disclosed in DE-OS 42 39 845. This discloses a method of making adjustments at the headbox of a web-forming machine to provide a web of relatively uniform density and layer height utilizes a headbox with a plurality of transverse sections, each of which is provided with a web material at a variable flow rate and a variable consistency. The method includes the steps of measuring the layer height profile of the web, measuring the density profile of the web, and comparing a portion of the layer height profile with a corresponding portion of the density profile to determine whether there are corresponding deviations in the profiles. If there are deviations in the layer height and density profiles, the consistency and flow rate of the web material provided to the headbox are adjusted, based on deviations in the layer height profile and stock density profile and whether the deviations correspond with each other, in order to achieve a uniform stock density profile and layer height profile. The web is preferably dewatered by providing it to a dewatering apparatus, pressed and dried to provide a finished web, and rolled into a roll. The method may be used in connection with a paper machine to form a paper web.
The prior art discloses a flowbox or headbox that is constructed at least partially in sections arrayed in the cross-machine direction. The flowbox influences the paper stock suspension in the individual sections, particularly as to its consistency and its fiber orientation, by feeding dilution water, for example, to control the cross-machine direction grammage profile. Sectional flowboxes are disclosed in allowed U.S. application Ser. No. 08/662,980 now U.S. Pat. No. 5,707,495, and U.S. Pat. No. 5,466,340 whose contents are incorporated by reference herein.
The achievable level of the cross-machine direction profile quality feature, the grammage in the present example, using a cross-machine direction control system of this type, was limited, particularly by the fluctuations occurring during paper manufacturing, in the machine direction profile of the quality feature of the paper web. The main cause of machine direction fluctuations, if the cross-machine direction grammage profile is considered, are volume flow fluctuations and consistency fluctuations in the stock jet.
A traversing sensor that operates point by point, as in DE 42 39 845, to pick up the measured signal relating to the quality feature of the paper web cannot precisely ascertain the cross-machine direction profile of the quality feature. Instead, it is only possible to determine a signal that is composed of cross-machine and machine direction profile components. This results from the speed at which the paper web runs through the papermaking machine, which is generally greater than 1000 m/min.
The measurement signal, which includes mixed cross-machine and machine direction components, can be used for the control device for the open-loop/closed-loop control, if it is viewed as a cross-machine direction profile signal which is "averaged" over a certain machine direction section of the paper web. However, such averaging over a certain paper section leads to a very "noisy" cross-machine direction profile measured signal. An actual cross-machine direction profile is extracted by filters from this noisy cross-machine direction profile measured signal, and the extracted actual cross machine direction profile is used for control. The severe filtering of the cross-machine direction profile measurement signal enables control interventions of the cross-machine direction profile control system to be performed only infrequently or with too low an amplitude. Thus, control interventions according to the prior art are performed only after a very long time delay in relation to the measurement, for example only after five minutes after picking up the measured signal. At a web production speed of 1500 m/min, for example, this means that the paper web has travelled a distance of 7500 m before a control intervention is performed. This produces problems, for example, the transient behavior of the control system is severely slowed and the attainable cross-machine direction profile is limited by the random component on account of the machine direction fluctuations in the cross-machine direction profile.
In order to solve this problem, German Application DE-AS 20 19 975 proposes using both a stationary sensor and a sensor traversing over the web width for sending the thickness measurement of a web. The two sensors pick up a thickness profile of the web which is passing by. The stationary sensor measures the thickness profile in the machine direction at one point along the web, while the traversing sensor measures the thickness profile on a path running obliquely over the web. The obliquely running profile contains both components of the pure cross-machine direction profile and components of the pure machine direction profile. By subtracting the machine direction profile from the oblique profile, the pure cross-machine direction profile is then determined. Disadvantages of this method or device are that too much time elapses for complete measurement of a cross-machine direction profile and that the thickness of the paper web can only be measured at the end of the papermaking machine. It produces an excessively long or sluggish reaction time for the control system.